Wireless network operators today invest considerable amounts of manual effort in planning, configuring, optimizing, and maintaining their cellular systems. Such efforts can consume a great part of the operators' “OPEX”, i.e. their operational expenditures.
Thus, an important wish from the operators' side is a reduction of the manual effort need for the deployment, configuration, and optimization phases of their cellular systems, both for existing systems and future systems. Such a reduction can be obtained by means of automation of the tasks typically involved in planning and operating a system.
A viable method for such automation is to use the user equipments, UEs, in the system to measure and report a number of system parameters. In such a context, it becomes important to have a reliable method for determining a UE's location. Determining the angle between a UE and a base station, usually the base station of the UE's serving cell, is important in this context, the angle in question sometimes being referred to as Angle of Arrival (as seen from the UE) or Angle/Direction of Departure (as seen from the base station).
There are Direction of Departure estimation methods known in the prior art. For example, if accurately synchronized in time, signals received from several antennas or antenna elements at a base station can be correlated to determine the Direction of Departure. Known methods for this include subspace methods, which have a numerical efficiency. Such methods are also possible to use based on less accurate measurement, for example in relation to less accurately time aligned antenna elements or less detailed measurements such as power measurements. It is also known in the art that there is a relation between antenna gains to multiple antenna elements and the received power level at a mobile terminal. Antenna information and mobile terminal power measurements can thus lead to angle of arrival estimates, either one unique or multiple ambiguous candidate angles.
The methods mentioned for determining the Direction of Departure may work well in theory, but “real life” UEs do not present bias-free measurements. For example, a UE will often have difficulties in accurately measuring a weak signal in the presence of a very strong signal, which leads to inaccurate signal strength difference estimates, and possibly also a situation in which the weaker signal is not detected at all by the UE.
The accuracy of methods used for determining the Direction of Departure between a UE and a base station may also be improved upon if statistics for the Direction of Departure can be arrived at in an accurate manner.